Obviously, Cain was afraid of others slaying him and he had to have found a wife among and directed the building of a city for a significant number of what we call people.

13And Cain said unto the LORD, My punishment is greater than I can bear.

14 Behold, thou hast driven me out this day from the face of the earth; and from thy face shall I be hid; and I shall be a fugitive and a vagabond in the earth; and it shall come to pass, that every one that findeth me shall slay me.

15 And the LORD said unto him, Therefore whosoever slayeth Cain, vengeance shall be taken on him sevenfold. And the LORD set a mark upon Cain, lest any finding him should kill him.

16 And Cain went out from the presence of the LORD, and dwelt in the land of Nod, on the east of Eden.

17 And Cain knew his wife; and she conceived, and bare Enoch: and he builded a city, and called the name of the city, after the name of his son, Enoch.

Cain, being the seed of Nachash, the “shining enchanter” (see The Old Testament Hebrew Lexicon;Bible Study Tools) and Eve, the first Adamic female, would have possessed a high level of intelligence and abilities, and was quite likely superior in that sense to those people for whom he directed the building of a city, which he named for his son Enoch. This city is also known to archaeologists as Erech and Unuk.